In recent years, optical lenses molded to have a predetermined shape have been used as lenses for digital cameras and the like, and concave lenses as well as biconcave lenses have become common optical components. In light of precise and large-scale manufacture of these optical lenses, methods in which a preliminary molding is produced first by molding a molten glass material to have a shape as approximate as possible to the shape of the final optical element, and the preliminary molding is supplied to a final mold and subjected to hot working have been generally employed.
According to this method, reduction in amount of deformation of the optical glass in hot working, and shortening of contact time with the mold may be achieved, thereby resulting in decrease in defective molding and prolongation of lifetime of the mold. In addition, effects of shortening of molding tact time may be expected.
Thus, methods of manufacturing an optical element in which preliminary molding is conducted once have been disclosed in various documents. However, there do not exist many which refer to the shape of the preliminary molding.
On the other hand, a glass preliminary molding having a particular shape has been disclosed (for example, Japanese Patent Application First Publication No. H05-213622). In this document, a glass member previously cut to have a predetermined size is placed on a supporting member having a circular opening, and is once heated for softening to approximate to the catenary curve. Disclosed is a method of obtaining a glass preliminary molding having a biconcave face by further placing this glass member approximated to the catenary curve in a mold with the upper and lower mold halves having a convex surface, followed by carrying out press molding.
However, because this glass member approximated to the catenary curve has a convex shape on one face, it is unsteady in precise press molding. Therefore, defective molding is liable to occur. Furthermore, there is a method in which a glass material is cut away from a glass block material, and is then ground and polished to give an optical element having a biconcave face. However, in this case, a long time may be required for carrying out many steps in an alternating succession, which may consequently result in increase in cost.
Meanwhile, a glass preliminary molding previously having a convexo-concave or biconvex shape was disclosed (for example, Japanese Patent Application First Publication No. H09-12318).
However, the glass preliminary molding described in the document is based on requirement of change of the mold for preliminary molding to appropriately correspond to the final shape of the optical element, so to speak; therefore, a certain glass preliminary molding cannot adapt to the pressing for every final shape. Therefore, consideration of the lifetimes of molds for manufacturing the preliminary molding has become necessary, and thus the method is far from being envisaged as an ultimate solution in light of reduction in cost.